1. Field of the Invention
The present invention relates to the technical field of reducers, which transmit drive power from an input shaft to an output shaft at a reduced rate.
2. Description of the Related Art
Conventionally, reducers contain a lubricant such as a grease inside for friction reduction and for cooling purposes (see Japanese Patent Application Laid-Open No. 2006-38108).
A grease or an oil is used as the lubricant. It is generally known that an oil is better than a grease in the friction reduction and cooling performances.
Making various parts (gears etc.) constituting the reducer can achieve a goal to make the reducer compact, but it also results in a smaller output capacity of the reducer. There is, therefore, another approach to make the reducer more compact, not by making the components smaller but by making the internal space of the reducer (where a lubricant is contained) smaller, so that the output power of the reducer is maintained (or increased).
To keep high output power of the reducer, oil lubrication is more advantageous than grease lubrication. On the other hand, if the reducer is designed to have a smaller internal space to make the reducer more compact, the pressure inside easily rises during the operation of the reducer and a lubricant contained inside more easily leaks out. This is presumably because a smaller internal space provides a smaller buffer space for accommodating the pressure buildup.
To prevent such a pressure buildup, for example, an “air release hole” provided to the reducer would suffice. However, the reducer may not necessarily be used always in the same orientation during the operation. A mere “air release hole” would let oil leak out if the orientation is changed during the operation.